Flaming rainbows: Pretty, but dangerous | Science News for Students

Flaming rainbows: Pretty, but dangerous

Bowls of burning chemicals across a desktop look great, but can cause injury
Nov 19, 2015 — 7:00 am EST
American Chemical Society

Although they look like fun, flaming demonstrations can be dangerous. Now, teachers’ and science groups have come out against one popular fiery demo, and in favor of a safer alternative. 

American Chemical Society

Students walking into a science class at W.T. Woodson High School in Fairfax, Va., on October 30 thought they were going to see a fun, fiery demonstration. But instead of awe-inspiring chemistry, five got whisked to the hospital for burns on their faces, heads and arms.

The culprit? A demonstration called the “flame rainbow.”

Teachers start by placing a set of bowls containing metal salts across a table top. They soak each salt in methanol — a toxic, flammable alcohol — and then light it on fire. When done properly, each salt forms a lovely blazing flame in a different color. Arranged in the right order, they resemble a rainbow of fire.

But when the demo goes wrong, results can be disastrous. Now, two science groups have decided they had better issue warnings. For years, the American Chemical Society, or ACS, has been issuing warnings about the demonstration. Last week, it released a video showing a safer alternative. The same week, the National Science Teachers’ Association issued a safety alert, begging teachers not to use methanol. Keep the flames, they say. Just leave the methanol behind.

DANGEROUS CHEMISTRY Following accidents with methanol flame rainbows, the Chemical Safety Board released this video to let people know about the dangers. USCSB

The chemistry class in Virginia is not the first to have flaming rainbows go awry. One accident at a Denver high school in 2014 produced a jet of fire that shot 15 feet and hit a student in the chest. “Since the start of 2011, I’ve found 18 incidents injuring at least 72 people,” says Jyllian Kemsley.  This chemist is a reporter for the ACS magazine Chemical and Engineering News, based in in Washington, D.C.

“You’re using methanol to burn something,” Kemsley notes. So these fires are perfectly predictable, she says. With such a highly flammable liquid, it isn’t a surprise that things could get out of control. But it never has to, she adds, because this demonstration doesn’t require methanol at all.

How the rainbow flame works

Teachers light this colorful fire by igniting metal salts soaked in methanol. These metal salts are made from pairs of ions — atoms with electrical charges. One ion in each pair is a metallic element — such as copper and potassium. The other ion — sulfur or chloride, for example — has an electrical charge that balances out the metal. This pairing creates a salt with no net electrical charge.

The color in the burning salts comes from the energy contained in their electrons — the negatively charged particles that move around the outer edges of atoms. These electrons become excited when energy is added — for instance, when you set the salt on fire. As the salt burns, the extra energy is lost — as light.

The color of that light depends on the amount of energy being released. Lithium salts burn a bright red. Calcium glows orange. Basic table salt burns yellow. The flames coming off of copper are bluish-green. Potassium burns violet.

With all of these salts burning different colors, all teachers have to do is line them up in the order of colors in a rainbow — red, orange, yellow, green, blue, indigo and violet.

“It’s a nice way to visualize what can seem abstract — what electrons are doing in an ion,” says Kemsley. The principle also can be used as an experiment. Students can light an unknown substance and record its color. That hue can help them figure out what is in the substance. “If you burn it and it comes up green, there’s a chance you’ve got copper in there,” Kemsley explains. “I think there’s value in doing that.” 

From demonstration to danger

The problems usually occur when the flames begin to go out. “You’ve got them all burning, and one goes out,” explains an industrial chemist and blogger that goes by the name of “Chemjobber.” Because he works in industry, he prefers not to give his name. But he has written many blog posts about the dangers of the rainbow-flame demos.

As the flames go out, the students want to see more, he explains. “The teacher goes and pulls out the bulk bottle of methanol.” For safety, the teacher should pour some of the methanol into a tiny cup, and then add it onto the flames. But when in a hurry, a teacher may sometimes pour the liquid directly from the bottle.

Methanol burns with no color. It can be hard to tell where the fire is and where it’s going. If the experiment goes wrong, Chemjobber says, “There’s a flash effect. The flame goes back into the bottle [of methanol] and shoots out at the students” nearby.

“People need to be really aware of the worst case scenario,” says Chemjobber. “The worst case is really bad.” He stresses that these are not minor burns such as those from a hot pot. “It’s skin grafts and surgery and a trip to the burn unit. It’s going to take a long time to heal.”  High school student Calais Weber was burned by a rainbow flame demonstration in 2006. As part of her treatment, she had to be put into a medically-induced coma. She remained in the hospital for two and a half months.

Keep the rainbow, ditch the methanol

There are safer ways to do the rainbow flame experiment, as the new ACS video illustrates. Instead of pouring methanol into dishes of metal salts, teachers can dissolve the salts in water. Then they leave the ends of wooden sticks in the solution to soak overnight. Those sticks absorb the salty solution. When the teacher (or student) puts the ends of the wooden stick over a bunsen burner — a controlled-flame gas burner used in laboratories — the salts will transform the flame’s color.

A SAFER RAINBOW This new video from the American Chemical Society demonstrates a much safer way to demonstrate the rainbow colors of various burning salts. No alcohol needed. American Chemical Society

It’s just one color at a time instead of a simultaneous rainbow. Still, Chemjobber argues that this version “is more tactile.” It lets people handle the sticks and burn them themselves. The downside: “It’s not as mesmerizing.” But if teachers feel compelled to go for the dramatic full- rainbow effect, he says, they should use a chemical hood, with plenty of protective equipment.

Teachers, Kemsley says, must “think through what can go wrong.” They need to ask themselves: “What’s the worst case scenario?” If the worst case involves a flaming fire of methanol, it’s probably best to try something else.

Students also need to ask themselves if the teacher is doing the experiment safely. If a student sees a situation that seems unsafe — such as a large, open bottle of methanol near open flames — it’s a good idea to speak up, and see if there’s a way to put the methanol in the cabinet during this demonstration. Otherwise those students should step back. Way back.

Power Words

(for more about Power Words, click here)

atom   The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and neutrally charged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

bunsen burner  A small gas burner used in laboratories. A valve allows scientists to precisely control its flame.

coma   A state of deep unconsciousness from which a person cannot be awakened. It usually results from disease or injury.

copper  A metallic chemical element in the same family as silver and gold. Because it is a good conductor of electricity, it is widely used in electronic devices.

electric charge   The physical property responsible for electric force; it can be negative or positive.

electron  A negatively charged particle, usually found orbiting the outer regions of an atom; also, the carrier of electricity within solids.

ion   An atom or molecule with an electric charge due to the loss or gain of one or more electrons.

lithium   A soft, silvery metallic element. It’s the lightest of all metals and very reactive. It is used in batteries and ceramics.

methanol   A colorless, toxic, flammable alcohol, sometimes referred to as wood alcohol or methyl alcohol. Each molecule of it contains one carbon atom, four hydrogen atoms and an oxygen atom. It is often used to dissolve things or as a fuel.

molecule An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

potassium   A soft, highly reactive metallic element. It’s a nutrient important for plant growth, and in its salt form (potassium chloride) it burns with a violet flame.

salt     A compound made by combining an acid with a base (in a reaction that also creates water).

scenario    An imagined situation of how events or conditions might play out.

tactile An adjective that describes something that is or can be sensed by touching.